In recent years, photocatalytic materials have gained considerable attention as materials which are hydrophilized by their exposure to sunlight and perform a self-cleaning function with the aid of rainfall when applied to building exterior surface. They have also gained attention as environmentally friendly materials which decompose toxic gases such as NOx.
For example, Japanese Patent Laid-Open No. 10-195333 discloses that a coating film capable of decomposing NOx is formed by applying a coating containing water-soluble silicate, a hardening agent and photocatalytic powder (titanium dioxide or zinc oxide) onto the surface of tunnel wall or guardrail and heat-treating the same.
Japanese Patent Laid-Open No. 10-237354 discloses that a building material superior in self-cleaning properties is obtained by applying a coating containing lithium silicate and titanium dioxide onto a building material and heat-treating the same.
In recent years, there has been an increasing tendency to use a water-based coating rather than a solvent base coating from the viewpoint of work environment, effect on surroundings and odor. As a result, water base photocatalytic coating materials to be applied on building exteriors etc. as described above have also been proposed.
For example, Japanese Patent Laid-Open No. 10-195369 discloses a coating composition which is obtained by blending a photocatalyst and a perfluorocopolymer in the emulsion state.
Japanese Patent Laid-Open No. 10-279886 proposes a coating composition which is a silicone emulsion containing a photocatalyst and a fluoro group.
Further, in the fields of building construction and coating, dirt and stains on building exteriors, outdoor structures and the coating film thereon have been posing problems with the increasing environmental pollution. Dust and particulate matter suspended in the air accumulate on the roofs and exterior walls of buildings in fine weather. The accumulations are washed away by rainwater when it rains and flow down the exterior walls of buildings. Further, airborne soot and dust are carried by rain water in rainy weather and flow down the exterior walls of buildings and the surface of outdoor structures. As a result, contaminants are attached on the surface of the exterior walls and that of the structures along the routes rain waters have taken. Once the surfaces are dried, there appears dirt in stripes on the surfaces.
Dirt and stains on building external facings or building coating films are made up of contaminants of: combustion products such as carbon black, urban soot and dust, and inorganic matter such as clay particles. Such diversity of contaminants makes contamination preventive measures complicated (Kitsutaka, Y. “Accelerated Test Method for Contamination of Exterior Wall Coating,” Transactions of Structural Division, Architectural Institute of Japan, No. 404, October 1989, 15-24).
Previously, an idea was generally accepted that water-repellent coatings such as polytetrafluoroethylene (PTFE) were preferably used to prevent dirt and stains on building exteriors etc. as described above. In recent years, however, an idea has been generally accepted that the surface of coating films on building exteriors etc. should be as hydrophilic as possible so as to prevent accumulation of dirt of urban soot and dust, which contain a lot of hydrophobic components (Macromolecule, Vol. 44, May, 1995, 307). In such circumstances, a proposal has been made that building exteriors should be coated with a hydrophilic graft polymer (Newspaper, “Japan Chemical Week,” Jan. 30, 1995). According to the newspaper's report, the coating film of the hydrophilic graft polymer exhibits such hydrophilic nature that the contact angle of water on the coating film is 30 to 40°. However, the contact angle between inorganic dust, typified by a clayey mineral, and water is 20° to 50°, and such inorganic dust has an affinity for the graft polymer on which contact angle of water is 30 to 40° and is likely to attach on the surface of the graft polymer. Thus, the coating film of such graft polymer is not capable of preventing the dirt and stains of inorganic dust. In the mean time, a variety of hydrophilic coatings have been sold which are composed of acrylic resin, silicone-acrylic resin, water base silicone, block copolymer of silicone resin and acrylic resin, acrylic-styrene resin, sorbitan fatty acid ethylene oxide, sorbitan fatty acid ester, urethane base acetate, cross-linked urethane of polycarbonate diol and/or polyisocyanate, or polyacrylic alkyl ester. The contact angle between each of the above hydrophilic coatings and water is 50 to 70° at the most, and such coatings are not capable of effectively preventing the dirt and stains of urban soot and dust, which contain a lot of lipophilic components.
As means for resolving the above described problems, coating compositions containing a photocatalyst have been proposed. Photocatalyst-containing coating films are capable of making their surface hydrophilic when exposed to UV rays from outdoor light and keeping the contact angle of water on them 20° or less. Further, they have the effect of both inhibiting the propagation of molds and algae and removing toxic substances such as NOx and SOx.
Examples of coating compositions that contain a photocatalyst are those disclosed in: 1. WO 98/03607, 2. Japanese Patent Laid-Open No. 11-1659, 3. Japanese Patent Laid-Open No. 2002-69376 and 4. Japanese Patent Laid-Open No. 10-316937.
WO 98/03607 discloses a composition including: photocatalytic particles composed of metallic oxide; at least one selected from the group consisting of silica fine particles, a silicone resin film precursor capable of forming a silicone resin film and a silica film precursor capable of forming a silica film; and a solvent, wherein the concentration of the total amount of the above photocatalytic particles plus the above silica fine particles or precursor, in terms of silica weight, in the composition is 0.01 to 5% by weight. In the examples described in the above specification, tetrafunctional silane was used as the silicone resin film precursor capable of forming a silicone resin film. The specification states that to prevent the film from becoming opaque white due to irregular reflection of light and allow the same to be substantially transparent, the thickness of the film is preferably 0.4 μm or less. Further, in examples described in the specification, alcohol having high power of dissolving organic resins, such as methanol or propanol, was used together with water so as to dissolve tetrafunctional silane.
Japanese Patent Laid-Open No. 11-1659 discloses a coating composition which is obtained by dispersing a photocatalyst in fluorine resin and silica (or the precursor thereof) or silicone (or the precursor thereof). In the examples described in the specification, solvent base fluorine resin and solvent base silicone resin were used so as to produce a highly durable coating. Solvent base coatings are superior in weathering performance; however, when they are applied to an organic base which is susceptible to attack by solvents, such as emulsion paint, their weatherability may possibly deteriorate (troubles such as peeling and cracking may possibly occur) after application. In the examples described in the specification, the coating film was dried at 120° C., so the hardening conditions after coating may also affect the durability of the coating film in this technique.
Japanese Patent Laid-Open No. 2002-69376 discloses a self-cleaning coating composition which is obtained by mixing photocatalytic particles or the sol thereof into trifunctional silicone resin and/or trifunctional silicone resin precursor capable of forming a silicone resin film and a whisker (or mica, talc) and which provides a film such that the contact angle of water on its surface is decreased to 20° or less when it is exposed to light.
Japanese Patent Laid-Open No. 10-316937 discloses a coating composition which is obtained by dispersing photocatalytic particles in water base silicone emulsion resin containing a surfactant so that the photocatalytic particles constitute 5% by weight or more of the composition. The specification states that the composition can be applied directly onto plates whose surface has a coat of organic paint. And in the examples described in the specification, direct application of the composition onto such plates was performed and evaluated.
The water-based coatings disclosed in Japanese Patent Laid-Open Nos. 10-195333 and 10-237354 have poor wettability to base materials having hydrophobic substances on their surface, such as plastics and painted steel, and therefore their applications are limited to glass, wood and metals.
The water-based coatings disclosed in Japanese Patent Laid-Open Nos. 10-195369 and 10-279886 have improved wettability to base materials having hydrophobic substances on their surface, such as plastics and painted steel; however, assuming that they are used outdoors, the contact angle of water on them right after their application is still too large. As a result, they could not perform their self-cleaning function with the aid of rainfall right after their application.
From the viewpoints of environmental burden, consideration for and safety of builders or a building site and its vicinity, replacing solvent base coatings by water base ones is the problem which we have to face.
Self-cleaning coatings for use in exterior walls are required to have good weatherability. To enhance the weatherability of a coating, particularly of a coating containing a photocatalyst, the thickness of the coating film has to be increased to such an extent that radicals generated from the photocatalyst by exposing the coating film to ultraviolet rays do not reach the layer underlying the coating film (several μm or more) and thereby protect the underlying layer from the activity of the photocatalyst. But on the other hand, with the increase in thickness of the coating film, cracks are more likely to occur on the film surface. Thus, how to make weatherability and prevention of cracking be compatible with each other is a problem with self-cleaning coatings containing a photocatalyst.
In repainting existing walls, organic primers are often used. These primers are composed of acrylic emulsion and therefore susceptible to attack by strong solvents. Accordingly, when intending to apply a coating directly onto such organic bases, a water-based coating is desirably used.
As aforementioned, WO 98/03607 discloses a coating composition in which tetrafunctional silane is mainly used as a silicone resin film precursor. However, when a film containing tetrafunctional silane is formed to thickness of several μm so as to impart the film with weatherability, cracks are likely to occur in the film.
Further, when the above coating composition is applied directly onto an organic base, the alcohol solvent attacks the organic base because of its high solvency power against resin, which may cause cracks or peeling of the film right after the application of the coating composition.
The inventors of this invention proposed in Japanese Patent Laid-Open No. 2002-69376 a photocatalytic coating composition which includes a blend of: a trifunctional silicone resin and/or a trifunctional silicone resin precursor capable of forming a silicone resin film; and whisker so as to provide a self-cleaning coating composition that has both weatherability and difficulty in causing cracks, but they could not make the composition a water base one. Further, the coating composition disclosed in Japanese Patent Laid-Open No. 2002-69376 was so hard that it needed an appropriate intercoat when applied to an organic material to be coated.
In the examples described in Japanese Patent Laid-Open No. 10-316937, coating films of 1 and 20 μm were formed on the respective plate whose surface has a coat of organic paint and the adhesion etc. of each coating film was evaluated. The evaluation showed that the adhesion was not sufficient for the coating film 20 μm thick directly applied onto the plate whose surface has a coat of organic paint. The specification did not disclose any data on the films' weatherability.
No prior art has disclosed a photocatalyst-containing and stain-proofing performance-exhibiting water-based coating composition that provides a coating film not only having good adhesion to organic materials as objects to be coated even when its coating is performed at ordinary temperature, but also having good weathering performance.
As described above, there has been no water base photocatalytic coating composition that has good adhesion to a substrate, does not cause cracks when used outdoors and has good weathering performance, even when it is formed into a film having a thickness sufficient to intercept ultraviolet light.
This invention has been made in the light of the above described circumstances. Accordingly, an object of this invention is to provide: a photocatalytic coating material which poses no problem in terms of work environment, effect on surroundings and odor, which can be applied onto base materials having hydrophobic substances on their surface, such as plastics and painted steel, and which allows, when it is formed into a film on base materials having hydrophobic substances on their surface, the formed film to have firm adhesion to the base materials, allows the contact angle of water on the surface of the film to be small even immediately after the application of the composition, and hence the film to perform its self-cleaning function with the aid of rainfall right after its use, and also allows the above described state of the film to be kept for a long time by exposing the film to sunlight; a photocatalytic composite material formed by applying the above photocatalytic coating material onto a base material having a hydrophobic substance on its surface; and a method for producing the same.
Another object of this invention is to provide a photocatalyst-containing and stain-proofing performance-exhibiting water-based coating composition that provides a coating film not only having good adhesion to organic materials as objects to be coated even when its coating is performed at ordinary temperature, but also having good weathering performance and therefore causing no cracks when used outdoors.